The present invention generally relates to improving the thermal performance of chip to heat-spreader interconnections, specifically as used with ball grid arrays. As is known, modem integrated circuit packages often times include semiconductor chips bonded to a metal heat-spreader or sink. Typically, the metal heat-spreader is attached on or to a circuitized chip carrier made from an epoxy resin or other plastic. An opening in this circuitized carrier allows the back, or inactive, side of the chip to be bonded directly to the heat-spreader, typically with an epoxy. A glob of some other epoxy or resin is used to encapsulate the chip as well as any metal wires present for electrically connecting the chip to leads on the carrier.
The chip carrier/heat-spreader assembly can be provided with a ball grid array ("BGA"). As is known, a BGA allows a device to be connected to and communicate with other devices in an electronic system. Unlike dual-in-line packages, which use pins to connect a device to another circuit card, BGAs utilize an array of solder bumps to make this connection. Pin connections are difficult to make at the very high densities that are possible with BGAs. In addition, the circuit line length is shorter for BGAs than it is for pinned connections, which is of particular importance for SRAMs used as cache. Capacitance and the cost per I/O are other advantages of BGAs over PGAs.
Often, BGAs having their base made from plastic are chosen for applications in which thermal performance is important. Plastics are, generally, poor thermal conductors. To improve the cooling in a chip PBGAs may have a cavity that allows a chip to be mounted in a "chip down" orientation. This "chip down" orientation of a BGA provides for a direct connection of a chip/BGA assembly to a heat-spreader which as part of this direct connection is adhered to a top side of the carrier which holds the chip. Often, the heat-spreader may serve as the device's only heat dissipation element, but a heat-sink can be attached to the heat-spreader for enhanced performance.
In order to securely bond the chip to the metal heat-spreader an adhesive is used. Since the function of the metal heat-spreader is to dissipate heat generated by the operation of the chip this adhesive needs to possess a high thermal conductivity. The adhesive also must exhibit a high bond strength to the metal of the heat-spreader.
An adhesive must then exhibit the following properties: 1) good thermal conductivity; 2) good chip-to-adhesive bond strength and 3) good metal-to-adhesive strength. Comparatively few adhesives exist which exhibit each of these properties to the desired degree. With the trend for larger chips, both in terms of size as well as power consumed, these requirements become increasingly severe. Thus, the selection of a single adhesive to meet all of these requirements becomes very difficult.
Adhesives demonstrating high thermal conductivity are highly filled and generally provide poor mechanical adhesion between the chip and the heat-spreader. Moreover, heat-spreaders are typically made from copper that may have been plated with another metal or may have been treated with chloride to preserve its surface finish. These surface treatments often reduce the strength of the interfacial bond to the adhesive. To mitigate the resultant reduction in adhesions an additional layer of adhesive having better adhesion properties can be added. This added adhesive layer invariably increases the thermal resistance of the bond both by increasing the total bond thickness and by the additional interfacial resistance.
Accordingly, there is a need for a new and improved bonding system to be used with cavity BGA's that can accommodate higher stresses, without significantly increasing the package's thermal resistance, while being simple and inexpensive to produce.